Stabilized wye-wye transformers



Nov. 2, 1965 1. K. DoRToRT 3,215,951

STABILIZED WYE-WYE TRANSFORMERS Filed March 6, 1962 l 23 ZINVENTOK J4TE- 25- Amami Af Mercer 0.57.@ f/vx. 54eme. Cie-ee .SoFFr/v 22 23 24 53'l ,4

United States Patent O 3,215,961 STABILIZED WYE-WYE TRANSFORMERS IsadoreK. Dortort, Philadelphia,Pa., assignor to I-T-E Circuit Breaker Company,Philadelphia, Pa., a corporation of Pennsylvania Filed Mar. 6, 1962,Ser. No. 177,801 6 Claims. (Cl. 336-12) l My invention relates to anovel stabilizing structure for Wye-Wye transformers, and morespeciiically relates to a novel tertiary winding for wye-wyetransformers wherein there is a high leakage reactance between the noveltertiary and the main windings of the transformer.

It is well known in the art that a wye-wye power transformer must have atertiary winding to `stabilize the voltage neutral with respect tofundamental frequency and triple-n harmonic zero-sequence componentvoltages.

It is common practice to rate such tertiary windings at 35% of thetransformer kva rating, even though this winding only carries thedifference magnetizing current between the magnetizing currents of thevarious transformer legs. This has been necessary since the addition ofa delta connected tertiary greatly reduces the zerosequence impedance ofthe transformer so that a large short circuit current can flow in theevent of a phaseto-neutral short circuit on the transformer. The usualtype of tertiary carries approximately 1/3 of the short circuit ampereturns.

The principle of the present invention is to provide a novel tertiarywinding which has a high leakage reactance with respect to the mainwindings to thereby substantially decrease the current flowing in thetertiary winding during fault conditions, whereby a tertiary windingconstruction which is simple in nature and occupies little space ispermissible.

Accordingly, a primary object of this invention is to reduce the sizeand cost of Wye-Wye transformers.

A further object of this invention is to provide a novel inexpensivedelta connected tertiary winding for wye-wye transformers.

A further object of this invention is to provide a simple andinexpensive transformer construction for stabilizing the voltage neutralwith respect to fundamental frequency and triple-n harmoniczero-sequence' component voltages.

These and other objects of my novel invention will become apparent fromthe following description when taken in connection with the drawings, inwhich:

FIGURE 1 schematically illustrates a three-phase transformer core whichis to have windings connected on the legs thereof.

FIGURE 2 schematically illustrates the transformer of FIGURE l ascomprising a wye-wye connected system.

FIGURE 3 is a vector diagram of the voltages and magnetizing currents onthe transformer of FIGURE 2 under ideal ux conditions.

FIGURE 4 is a vector diagram of the magnetizing currents and voltages ofthe core of the transformer of FIGURES 1 and 2 for practical conditionsof unbalance.

FIGURE 5 illustrates the connection of a delta connected tertiarywinding for the transformer of FIGURES 1 and 2 to provide stabilizationof the voltage neutral of the system.

FIGURE 6 shows a schematic illustration of my novel inventive structure.

FIGURE 7 shows a modification of FIGURE 6.

FIGURE 8 shows a schematicview of a three-phase transformer core withwindings thereon which is provided with a delta connected tertiarywinding in accordance with FIGURE 7.

FIGURE 9 is a top view of the transformer of FIG- URE 8.,

A ceive wye-wye connected windings.

ICC

FIGURE 10 is a side cross-sectional view of FIGURE 8 taken across thelines 10-10 of FIGURE 8.

FIGURE l1 shows a modification of the tertiary Winding of FIGURE 6.

FIGURE 12 shows a further modication of the structure of FIGURE 1l.

FIGURE 13 is a perspective view of a part of the transformer core ofFIGURE 1 when modified in accordance with a further embodiment of theinvention.

FIGURE 14 shows a still further embodiment of the invention in themanner illustrated in FIGURE 13.

FIGURE 15 shows the manner in which la plurality of turns can be usedfor the tertiary.

Referring now to the figures, FIGURE 1 illustrates a typicalthree-legged core-type transformer which will re- Thus, the core iscomprised of an upper and lower yoke and 21, respectively, which areconnected in any desired manner by core legs 22, 23 and 24. Each of thecore legs 22 through 24 receive primary and secondary windingsschematically illustrated by coil structures 25, 26 and 27 respectively.

The well-known schematic diagram for this system is illustrated inFIGURE 2 as comprising a wye connected primary winding 28 which isconnected to a three-phase input line comprising lines 29, 30 and 31 anda Wye connected secondary 32 which is, for example, connected to outputlines 33, 34 and 35.

From the construction of the three-legged core of FIG- URE l, themagnetizing currents required by the two outer legs 22 and 24 of thecore will be considerably greater than that of the center leg 23. Thatis, the magnetic reluctance from the common flux point at the center ofthe bottom yoke 21 to the common linx point at the center of the topyoke 20 and through the outside core legs 22 and 24 will involve twolengths of iron and two joints which are not present in the path throughthe center leg 23. Other differences will exist between the three legs22, 23 and 24 due to uncontrollable variations in manufacture.

FIGURE 3 shows a vector diagram of the phase voltages and magnetizingcurrents in the transformer of FIG- URES 1 and 2, assuming that thefluxes between the various legs are completely balanced. Similarly, FIG-URE 4 shows a vector diagram of the phase relationship of unbalancedmagnetizing currents with respect to the resulting unbalanced phasevoltages. Y

Because of the unbalanced flux paths in the transformer of FIGURE l asmet in practice, the three magnetic paths will not have the samemagnetic reluctance `so that the voltage vectors cannot remain balanced,as shown in FIGURE 3. Moreover, and since the system involved is athree-wire ungrounded system, the three magnetizing currents of FIGURE 4must add up to zero vectorially, and are, therefore, restrained fromassuming values that would permit balanced voltages. Therefore, thewinding 26 on the center leg of the transformer of FIGURE 1 may have agreater voltage than the windings on the two outside legs 25 and 27respectively, because the ux density in the center leg will be greater.

In consequence of this, and from the vector diagrams of FIGURE 4, boththe voltage and flux systems will have zero sequence components. Thezero sequence flux must return from the upper yoke 20 to the lower yoke21 outside :of the core it is traversing through the insulation medium,structural members, and so on. If now a delta connected tertiary Windingis built into the transformer, a circulating current will flow in thetertiary which is equivalent to the zero-sequence magnetizing currentrequired tlo balance the fluxes in the three legs of the core,

r 3 and restore the voltages to the balanced condition of FIGURE 3.

This is schematically illustrated in FIGURE 5 for the schematicallyillustrated core 40 which has Wye-connected primary windings 41, 42 and43 wound thereon, which have magnetizing currents in the direction ofthe arrow to the right of the windings, and which has secondary windings44, 45 and 46 wound thereon. The delta connected tertiary then includeswindings 47, 48 and 49 which will conduct the magnetizing currents im,which is the zero-sequence magnetizing current required for fluxbalance.

From the above, it would be initially thought that the delta connectedtertiary winding would present no problem in the manufacture of thetransformer, since it only carries a dilference magnetizing currentwhich is a -relatively small current. In practice, however, thezero-sequence impedance of a wye-wye transformer without a tertiary isvery large, so that a phase-to-neutral short circuit on the secondaryw-ould draw very little current. With a delta connected tertiary,however, the zero-sequence impedance of the transformer is greatlyIreduced so that a large short circuit current can now flow. However,since there can be no Zero-sequence current in the primary transformerwinding, the equivalent zero-sequence ampere turns must appear in lthetertiary, and would necessarily amount to 1/a of the ampere turns of thesecondary short circuit.

Therefore, to prevent damage to the tertiary winding, it must have a fargreater copper cross-section than would be required for i-ts normaloperation. In practice, the tertiary winding is rated at 35% of thetransformer kva. Therefore, the cost of the addition of these windingsin material and labor is appreciable, and the transformer core size isgreatly increased so that it can accommodate the extra winding andinsulation to again increase the total cost of the transformer.

The tertiary winding further serves to correct harmonic conditionswithin the transformer, due to 4the variations of the flux from asinusoidal shape because of n-on-linear transformer steel. While thisanalysis can be rigorously given, it is well known to the art, andresults in the existence of a flux in the three core legs of thetransformer which will induce a peaked voltage conguration, whereby thedifference between a sinusoidal voltage and the distored secondaryvoltage has the effect of producing a circular precession of thepotential of the neutral point of the windings about the virtual voltageneutral in the vector diagram of FIGURE 3.

The tertiary winding can be shown to provide a path for thezero-sequence third harmonic currents land Sn harmonic currents, so thatthe winding neutral is stabilized with respect to both the fundamentalfrequency, as discussed fully above, and the triple-n harmonics.

It is, therefore, seen that a delta connected tertiary winding is anecessity in Wye-Wye transformers.

The principle of the present invention is to provide a simpleinexpensive construction for the tertiary wherein a high leakagereactance exists between the tertiary and the main windings of thetransformer to decrease cur: rent flow in the tertiary during faultconditions and, thus, permit the use of small cross-sectional copper forthe 'tertiary windings, and to accomplish this without anyI significantincrease in the dimensions and weight of the iron core and the mainwindings.

One manner in which the tertiary winding can be -produced in accordancewith the invention is schematically illustrated in FIGURE 6 which showscore legs 22, 23 and 24 of FIGURE 1 in cross-section.

In FIGURE 6, two single-turn tertiaries are contemplated, one at the toptransformer yoke within the Windows of the transformer and the other atthe bottom of the transformer wi-thin the windows. Such a constructionwill be seen to only slightly increase the height of the core windowswithout any increase in width. Moreover, no winding space is required sothat the tertiary can be assembled within the transformer for verylittle expendi- -ture in labor and material cost.

In FIGURE 6, the tertiary is formed of three windings 50, 51 and 52 ofone turn each and connected in delta. Obviously, this would be theequivalent of a simple loop around the core legs, shown in FIGURE 7 asthe single loop 53.

A transformer using the concept of FIGURES 6 and 7 may be manufacturedas shown in FIGURES 8, 9 and 10 the core 60 having windows 61 and 62receives three windings 63, 64 and 65 which are the correspondingprimary and secondary windings of the transformer.

The tertiary is then made of flat conductive bands or sheets 66 and 67which encircle the core in the manner shown in FIGURE 7. Each of sheets66 and 67 are sandwiched between insulation sheets 68-69 and 70-71respectively which insulates the delta connected tertiary windings 66and 67 from the remaining structure.

When using the construction of FIGURES 8, 9 and l0, the reluctance ofthe core legs of the transformer between the tertiary windings 66 and 67willbe relatively small,

as compared to the reluctance of the joints between the core legs andyoke external of the core legs. Therefore, most of the zero-sequenceflux, both fundamental and third harmonic, will leave and re-enter thecore in the areas of the yokes. Therefore, thelocation of single-turntertiaries just inside the Windows will be an effective way ofcancelling out these fluxes.

Moreover, and because of the high leakage reactance between tertiaries66 and 67 and the primary wind-ows, the tertiary need be designed onlyfor approximately 1/3 of the magnetizing ampere turns of thetransformer, as contrasted to 1/3 of the fault current due to a short onthe secondary winding.

In power transformers, the magnetizing current will generally be of theorder of 1% of the rated ampere turns, which, therefore, permitsconstruction of the tertiaries in the form of the at sheets or foils 66and67 which can be between the two relatively thin layers of insulation.It will be noted that the insulation sheets 68, 69, 70 and 71 need notbe greater than the turn-to-turn insulation in the main transformerwindings.

In the transformer of FIGURES 8, 9 and 10, a straight loop concept,illustrated in FIGURE 7, was utilized. It may, however, be advantageousin certain applications to use the configuration of FIGURE 11 wherein aconductive sheet is stamped with two protrusions which enter betweenrespective adjacent core legs. An equivalent of this type of connectionis shown in FIGURE 12 where the conductive sheet 82 has the slot in themiddle.

With this type of arrangement, it is possible to reduce the lluXleakages of the tertiary by magnetic flux which is not contained withinthe iron to thereby avoid the deviation of magnitude and wave shape ofthe current tlowing in the tertiary from that required for exactcompensation.

The tertiaries of FIGURES 6 through 12 could, of course, be constructedof a single loop made of wire, bar or sheet material, and could alsoutilize multiple turns arranged in a flat or pancake configuration, asshown in FIGURE 15 for windings 83a, 83h and 83e. This reduces theeffect of excessive impedance in the interconnecting straps by reducingthe current in these interconnections.

As a further embodiment of the invention, the transformer core of FIGURE1 is partially illustrated in schematic form in FIGURE 13 wherein acopper sheet band encircles the yoke 20. This type of arrangement willprevent the escape of flux from the sides of yoke 20.

Alternative to this, and as shown in FIGURE 13, the copper band 90 couldbe replaced by a conductive box 91 which completely encloses the yoke 20to prevent the passage of zero-sequence flux in any direction.

In applications of wye-wye transformers of the type to which theinvention is applied, such transformers are often used in rectifiersystems requiring the double wye connection with an interphasetransformer arrangement. The high zero-sequence impedance afforded bythis systern will materially reduce arc-back type faults in mercury arcrectifers with their destructive effects on the rectifier transformers.The wye-wye power transformer is also required at times for matchingphase angles of interconnected systems, or for impedance grounding ofsubstation transformers. In any of these applications, the novelarrangement for stabilizing such wye-wye transformers as discussed abovecan be utilized.

When utilizing the present invention in an application using overloadrelays to protect the transformer against a phase-to-neutral fault onthe secondary, some care should be taken in the connection of therelays. Thus, undervoltage relays connected from line to neutral on allthree phases, or phase balance voltage relays should be used to provideadequate protection. These relays could be used on the primary orsecondary side of the transformer, either with or without potentialtransformers, as required.

Moreover, and in accordance with the invention, an overcurrent relay canbe connected in the tertiary loop where the relay and its connectionshave a very low impedance.

Although I have described preferred embodiments of my novel invention,many variations and modifications will now be obvious to those skilledin the art, and I prefer therefore to be limited not by the specificdisclosure herein but only by the appended claims.

I claim:

1. In a three-phase transformer; a magnetic core having an upper andlower yoke connected by three core legs along respective coplanarjunctions between said upper and lower yokes and the respective ends ofeach of said three core legs, a respective primary winding and secondarywinding wound on each of said core legs, said primary windings beingconnected in wye, said secondary windings being connected in wye; atertiary winding for each of said core legs; said tertiary windingsbeing connected in delta; said tertiary windings being coupled to -saidprimary and secondary windings by a high leakage reactance; saidtertiary windings being formed of a sheet of conductive material; saidsheet of conductive material encircling said three said core legs in aplane slightly below the plane of said coplanar junction between saidthree core legs and one of said yokes.

2. In a three-phase transformer; a magnetic core having an upper andlower yoke connected by three core legs along respective coplanarjunctions between said upper and lower yokes and the respective ends ofeach of said three core legs, a respective primary winding and-secondary winding wound on each of said core legs, said primarywindings being connected in Wye, said secondary windings being connectedin Wye; a tertiary winding for each of said core legs; said tertiarywindings being connected in delta; said tertiary windings being coupledto said primary and secondary windings by a high leakage reactance; saidtertiary windings being formed of a sheet of conductive material; saidsheet of conductive material encircling said three said core legs in aplane slightly below the plane of said coplanar junction between saidthree core legs and one of said yokes; and a second set of tertiarywindings identical to said last mentioned tertiary windings andpositioned adjacent the other of said yokes.

3. In a three-phase transformer; a magnetic core having an upper andlower yoke connected by three core legs along respective coplanarjunctions between said upper and lower yokes and the respective ends ofeach of said three core legs, a respective primary winding and secondarywinding wound on each of said core legs, said primary windings being-connected in wye, said secondary windings being connected in wye; atertiary winding for each of said core legs; said tertiary windingsbeing connected in delta; said tertiary windings being coupled to saidprimary and secondary windings by a high leakage reactance; saidtertiary windings being formed of a sheet of conductive material; saidsheet of conductive material encircling said three said core legs in aplane slightly below the plane of said coplanar junction between saidthree core legs and one of said yokes; said sheet of conductive materialhaving a substantially negligible height, said tertiary winding having apredetermined magnetizing current and a predetermined number of ampereturns under short circuit conditions; the cross-sectional currentcarrying area of said tertiary winding being insuflicient to carry morethan the magnetizing current of said transformer and smaller than thecross-sectional current carrying area required to carry 1/3 the shortcircuit ampere turns of said transformer.

4. In a three-phase transformer; a magnetic core having an upper andlower yoke connected by three core legs, a respective primary windingand secondary winding wound on each of said core legs, said primarywindings being connected in wye, said secondary windings being connectedin wye; a tertiary winding for each of said core legs; said tertiarywindings being connected in delta; said tertiary windings being coupledto said primary and secondary windings by a high leakage reactance; saidtertiary windings comprising a band of conductive material encirclingthe external sides of at least one of said yokes.

5. In a three-phase transformer; a magnetic core having an upper andlower yoke connected by three core legs, a respective primary windingand secondary winding wound on each of said core legs, said primarywindings being connected in wye, said secondary windings being connected'in wye; a tertiary winding for each of said core legs; said tertiarywindings being connected in delta; said tertiary windings being coupledto said primary andsecondary windings by a high leakage reactance; saidtertiary windings comprising a conductive box enclosing the outer sidesof at least one of said yokes.

6. In a three-phase transformer; a magnetic core having an upper andlower yoke connected by three core legs along respective coplanarjunctions between said upper and lower yokes and the respective ends ofeach of said three core legs, a respective primary winding and secondarywinding wound on each of said core legs, said primary windings beingconnected in wye, said secondary windings being connected in wye; atertiary winding for each of said core legs; said tertiary windingsbeing connected in delta; said tertiary windings being coupled to saidprimary and secondary windings by a high leakage reactance; saidtertiary windings being formed of ilat coplanar windings connected inseries with one another; said flat coplanar windings encircling saidthree said core legs in a plane slightly below the plane of saidcoplanar function between said three core legs and one of said yokes.

References Cited by the Examiner UNITED STATES PATENTS 1,412,782 4/22Dwyer 336-5 2,779,926 l/57 Johnson et al 336-5 JOHN F. BURNS, PrimaryExaminer. E. JAMES SAX, Examiner.

6. IN A THREE-PHASE TRANSFORMER; A MAGNETIC CORE HAVING AN UPPER ANDLOWER YOKE CONNECTED BY THREE CORE LEGS ALONG RESPECTIVE COPLANARJUNCTIONS BETWEEN SAID UPPER AND LOWER YOKES AND TH ERESPECTIVE ENDS OFEACH OF SAID THREE CORE LEGS, A RESPECTIVE PRIMARY WINDING AND SECONDARYWINDING WOUND ON EACH OF SAID CORE LEGS, SAID PRIMARY WINDINGS BEINGCONNECTED IN WYE, SAID SECONDARY WINDINGS BEING CONNECTED IN WYE; ATERTIARY WINDING FOR EACH OF SAID CORE LEGS; SAID TERTIARY WINDINGSBEING CONNECTED IN DELTA; SAID TERTIARY WINDINGS BEING COUPLED TO SAIDPRIMARY AND SECONDARY WINDINGS BY A HIGH LEAKAGE REACTANCE; SAIDTERTIARY WINDINGS BEING FORMED OF FLAT COPLANAR WINDINGS CONNECTED INSERIES WITH ONE ANOTHER; SAID FLAT COPLANAR WINDINGS ENCIRCLING SAIDTHREE SAID CORE LEGS IN A PLANE SLITHTLY BELOW THE PLANE OF SAIDCOPLANAR FUNCTION BETWEEN SAID THREE CORE LEGS AND ONE OF SAID YOKES.